There are many situations where a generated ac signal requires regulating before it can be used elsewhere. The regulation may control frequency, voltage or current and may include control of voltage and current transients and steady state variations that may otherwise cause fluctuations in the ac waveform to be used elsewhere.
To provide a context for the present invention an intended application for the present invention will be described. This application is in dchp units that provide hot water and central heating in a domestic environment. Our International Patent Application No. PCT/GB03/001200 describes such a dchp unit comprising a Stirling engine. These dchp units are beneficial as, in addition to meeting a household's central heating and hot water requirements, they can also be used to generate electricity in an energy-efficient manner. The electricity so generated can be used either within the household or it may be sold back into the electrical grid supplying the household.
The electricity generated by the dchp unit must be tightly regulated to be suitable for supply onto a mains electrical grid. In addition, regulation is also usually required to suit the demands of domestic appliances connected to the dchp unit.
FIG. 1 shows the well-known Buck regulator that may be used to regulate a DC waveform. The regulator comprises a transistor that is switched, normally according to a pulse width modulation scheme, to provide a desired average output voltage. The inductor and capacitor smooth the pulsed output to leave only a minimal ripple on the DC voltage signal provided at the output. The regulator also includes a diode provided to act as a clamping diode (also known as a “flyback diode”) to protect the transistor from large reverse voltages generated by the inductor as it tries to maintain current flow when the transistor is switched off.
A pair of Buck regulators may be combined to provide a regulator for an ac supply. FIG. 2 reproduces such a regulator that is disclosed in EP-A-0,631,372. The regulator converts an ac input to provide a variable voltage dc output to power lights operated from a dimmer switch. A pair of transistors with associated diodes are provided, as indicated at A in FIG. 2, with one transistor modulating the positive half-cycle of the ac input and the other transistor modulating the negative half-cycle of the ac input. A pair of clamping diodes are provided, as indicated at B in FIG. 2, one for each half-cycle and biased appropriately, that are switched in and out of the circuit for the appropriate half-cycles by associated transistors. Thus, both positive and negative half-cycles are pulse width modulated, and the resulting output is smoothed by the inductors and capacitors.